Casting Java ArrayList<Custom Class> to ArrayList<Object>

Question

I am attempting to convert an ArrayList of class SomeClass to an ArrayList of class Object. This new ArrayList of Object will then be passed to a function. I currently have done the following:

// convert ArrayList<SomeClass> to generic ArrayList<Object>

Object[] objectArray = someClassList.toArray();
ArrayList<Object> objects = new ArrayList<Object>();

for (int i = 0; i < objectArray.length; i++) {
    objects.add(objectArray[i]);
}

someFunction(objects);

public void someFunction(ArrayList<Object> objects) {
    // do something with objects
}

Is there a more efficient or "standard" way of doing this? Is what I am doing "wrong" in the first place?

The purpose of converting it to ArrayList of class Object is that I have created an external library to process ArrayList of generic Objects.

Answer 1

If you are able to change the function's signature to taking an ArrayList<? extends Object> objects or an ArrayList<?> objects (or even better, List instead of ArrayList), you will be able to pass your ArrayList<SomeClass> directly.

The reason an ArrayList<SomeClass> is not an ArrayList<Object> is that an ArrayList<Object> would accept that you add() any kind of Object into it, which is not something you can do with an ArrayList<SomeClass>. On the other hand, an ArrayList<? extends Object> will allow you to retrieve elements from the list, but not add elements, so ArrayList<SomeClass> can safely be assigned to it.

Answer 2

Since you created the external library, I think it would be easier to modify the function signature to accept lists of any type. This can be accomplished using the unbounded wildcard ?:

public static void someFunction(List<?> objects) {
    // whatever
}

Then you don't need to make any conversions to call it:

public static void main(String[] args) {
    List<String> words = new ArrayList<>();
    someFunction(words);
}

Also, unless you have a good reason not to, it would be better to accept any List in someFunction instead of limiting your input to ArrayLists. This makes your code more flexible and easier to change in the future.